One example of a unit of the type described above can be found in, for example, WO 2013/172761, where the external unit and the internal unit each demonstrates in the embodiment shown in FIG. 11 two wings, achieving in this way four chambers in the unit, which chambers can be influenced by hydraulic pressure in order to rotate the moveable part relative to the fixed part.
A unit of the type described above can work with the absorption of extremely high external forces during rotation, and it can achieve this in all directions, for example axial, radial and tangential forces, and combinations of these. In order to avoid wear of the component parts, a bearing film of oil must be available between the contact surfaces of all component parts of the turnable unit.
One problem that then arises as a consequence of the necessity that the bearing film of oil between the contact surfaces have a high hydrostatic pressure between the contact surfaces and the necessity that these be able to move in two opposing directions is that it is not possible to use elastomer seals to seal between the mutually turnable parts. It is also a problem to obtain sufficiently high hydraulic pressure against the wings in order to be able to achieve high torques, as a consequence of leakage, given the lack of seals around the wings. In this way also the hydraulic efficiency of the unit will be low.
When the turnable unit is put under hydraulic pressure, it attempts to expand, which leads, in particular, to the side walls of the external part bulging outwards, in which way the play that is present between the wings of the internal parts and the side walls can increase markedly, in which way internal leakage within the unit increases, and in this way the hydraulic efficiency is noticeably reduced.
A doubling of the play between two parts leads to the volume of leakage between the two parts increasing by a factor of eight, such that, for a turnable unit that functions at high pressure, it is important to be able to maintain the play between the two mutually turnable parts as low as possible and at a pre-determined fixed level in order for the turnable unit to be able to work efficiently and in the manner intended.